The present invention relates to the reprocessing of irradiated nuclear fuel and particularly the dissolution of nuclear fuel pins.
Nuclear fuel pins consist of pellets of fissile material, e.g. UO2, contained in a cladding which is normally a zirconium alloy sold under the trade Zircaloy. A cluster of pins form a fuel assembly.
Commercial reprocessing of irradiated (spent) nuclear fuel uses the Purex process, which involves chopping up the pins of an assembly prior to dissolution of the fissile material in nitric acid. The pins must be chopped up to expose the pellets to nitric acid because the bulk zirconium alloy is resistant to attack by nitric acid, as is an oxide skin which irradiated zirconium alloy possesses. After the fuel has been dissolved the empty pieces of cladding (hulls) and other residues of the structure of the fuel assembly are encapsulated and disposed of.
Commercial reprocessing plants contain dissolvers based on one of two principles: batch operation or continuous operation using an apparatus with rotating mechanical parts.
In a typical batch dissolver the fuel is sheared into a large basket which is immersed in the dissolver vessel. After the fuel has been leached, the basket is removed from the dissolver using a crane and then tipped to transfer the hulls and other debris to the encapsulation plant. This system requires extensive mechanical handling of the dissolver basket which is heavy and needs a large amount of maintainable equipment in the shielded dissolver cell.
Continuous dissolvers shear the fuel into the segments of a wheel which is rotated inside a large vessel. The existence of rotating parts is a disadvantage and limits the geometry that can be adopted to a very large slab tank if the equipment is ever to be safe.
U.S. Pat. No. 4,246,238 discloses a batch action apparatus comprising a container in which is placed a basket filled with pieces of fuel pins, and to which pipe connections are fitted to feed in and remove solutions and gas. A disadvantage of the apparatus is the complexity of the sealing arrangement when the fuel pin pieces are loaded into the basket and when baskets are discharged from the apparatus, as well as the difficulty of containing radioactivity. In addition, use of such apparatuses would considerably complicate process automation and increase the volumes of radioactive solutions.
U.S. Pat. No. 4,230,675 discloses an apparatus which is used to make countercurrent contact between fuel pin pieces and leaching solution. The apparatus comprises an elongate cylindrical drum which rotates around its longitudinal axis and is divided by transverse partitions into a chamber for feeding in fuel pin pieces and removing solution situated at one end of the drum, and a chamber for exit of the cladding pieces and delivery of solution situated at the other end of the drum. Between these chambers there are several other chambers in which leaching of the fuel from the chopped cladding takes place. In each chamber there are elements which provide for movement of the fuel pin pieces on rotation of the drum. A disadvantage of the apparatus is the complexity of its design, and the presence of moving units and of units subject to intensive abrasive action caused by the fuel pin pieces. An apparatus of this design would therefore have a low level of reliability and require regular servicing and maintenance under conditions of high levels of radioactive contamination.
Moving away now from nuclear reprocessing, USSR author""s certificate No. 764698 discloses a mass-exchange device for solid phase treatment (primarily for the treatment of wood chips) which comprises a cylindrical casing inside which is a perforated helical ramp with a helical surface gradient of between 4 and 30 degrees. The perforations are in the form of apertures screened above and below by angled plates. Some of the apertures are fitted with nozzles protruding above the surface of the ramp. In a bottom part of the apparatus, which is separated from the annular container by a grid, there is a pulse generator. In use, solid particles are charged onto the ramp at its bottom end. Pulsed fluid from the pulse generator passes upwards through the apertures in the ramp and raises the solid particles above the ramp, creating a pulsing suspended layer in which the particles are processed by the fluid. The solid particles are moved upwards and discharged at the top of the ramp.
The apparatus of USSR author""s certificate No. 764698 could not be used with chopped nuclear fuel pins and is incompatible with a nuclear reprocessing plant. Amongst other things, chopped fuel pins have a diameter of between 8 and 20 mm, a length of between 25 and 100 mm and a weight of up to 70 g and would not be moved up the ramp of the apparatus as described.
A pulsation device which is designed for dissolving nuclear fuel pins is known from EP-A-358354 which comprises a V-shaped duct connected to a pulsation chamber containing nitric acid. Pieces of chopped fuel pin are loaded into the free end of one arm of the V-shaped duct. Compressed air in the pulsation chamber maintains nitric acid in the duct at a level which immerses most of the chopped pieces. The air pressure in the pulsation chamber is periodically released, resulting in most of the nitric acid in the duct leaving it. Compressed air is then again applied to the nitric acid in the pulsation chamber, causing acid to pulse into the duct such that leached hulls in the outlet arm of the duct are pushed upward to a discharge duct through which they leave the V-shaped duct.
An advantage of this apparatus is the simplicity of its design and the absence of moving parts. A disadvantage of the apparatus is the small volume of its process chamber and the low output of a single unit, with pieces of fuel pin spending a long time in the apparatus, as is essential for dissolution of the fuel. This is because the diameter of the V-shaped duct is restricted for criticality reasons, and the depth of the layer of fuel pin pieces in the duct is limited by the requirement that the pieces must move along steadily in a curving duct. To ensure that the required time is spent by the fuel pin pieces in the solution, therefore, a cascade of apparatuses of this type installed one after the other in sequence would have to be used. This would considerably complicate the apparatus layout, reduce the reliability of the equipment and also substantially increase the dimensions of the process areas.
The problem on which a first aspect of the invention is based, therefore, is to provide apparatus for use in the dissolution of nuclear fuel pins in which the apparatus can reliably be used to dissolve fuel contained in chopped cladding and to discharge hulls from the dissolver as well as require little maintenance and servicing, and which would not require a complicated apparatus layout or an excessive process area in the reprocessing plant.
The present invention provides a nuclear fuel dissolution apparatus comprising a perforated sloping ramp contained within a process chamber for containing solvent for the fuel and a pulsation member which in use creates pulses in solvent in the process chamber, the perforations being designed to direct pulses of solvent along and up the ramp, and a method comprising loading solvent into the process chamber, loading fuel pin pieces onto a lower region of the ramp and creating solvent pulses to transport the fuel pin pieces up the ramp to a discharge point where the cladding hulls are discharged from the ramp.
In preferred apparatus the ramp is spiral; the gradient of the spiral is preferably between 1 and 30 degrees and more usually between 1 and 20 degrees. Preferably, the process chamber is annular in cross section.
The gradient of the spiral in an upper zone of the process chamber may be greater than in a lower zone.
It is most preferred that the ramp is made out of flat blades, in which case the perforations of the ramp comprise inclined slits formed between the blades.
Preferably the angle between the plane of the blades and the horizontal is between 10 and 60 degrees.
In practice, the apparatus must be designed to have a so-called xe2x80x9ceversafe geometryxe2x80x9d, that is, to avoid a critical mass of material collecting which allows a self-sustaining fission reaction. For this reason, the apparatus will in practice be designed to control the total amount of fissile material which can accumulate in any one place and/or the geometry in which such accumulated fissile material is held. It is preferred for criticality reasons for the fissile material (chopped fuel pins) to be in elongate form rather than spherical. In a particular version of the method, each pair of adjacent blades makes a slit having a length (the dimension in the radial direction in the case of a spiral ramp in an annular chamber) of no more than 10 times the diameter of a fuel pin; such a design helps avoid an excessive accumulation of chopped fuel pins in the slit. More preferably, the slit length is between 5 and 10 times the diameter of a fuel pin.
In some embodiments the blades are made in the form of a trapezium and are fastened by the smaller end to a central blade support within the process chamber.
Preferred apparatus involves one or both of the features that the average width of the blades (extent of slit channel) is between 3 and 5 times the distances between them and that the distance between the plates at the outside wall of the container is 0.4 to 0.8 times the fuel pin diameter.
Preferably, the pulsation member comprises a pulsation chamber located coaxially within the process chamber. Normally, a neutron absorber is arranged between the pulsation chamber and an inside wall of the annular container.
In a particular version of the invention, the pulsation chamber, usually made in the shape of a cylindrical container, is located coaxially within the process chamber (which is usually annular) and communicates with a lower part of the process chamber by a an upwardly and outwardly directed duct (usually an annular slit). In preferred classes of the invention, a neutron absorber is situated between the pulsation chamber and an inside wall of the annular container.
The invention in another aspect provides an apparatus for the treatment of solid articles by liquid, comprising a container having an outer side wall of circular cross section, a spiral ramp located in the container, and a pulsator communicating with a lower part of the container, and also pipe connections for feeding in and removing pieces of fuel pin, solution and gas, characterised in that the ramp is made up of flat blades placed one after another along the spiral and forming between one another inclined slit nozzles. Such apparatus is particularly suitable for the chemical treatment of solid phase articles larger or heavier than the wood chips with which is concerned the prior art spiral pulsed fluid apparatus.
Usually, the process chamber of either aspect has an inner side wall as well as the inevitable outer side wall. The ramp in such chambers normally extends between the inner and outer side walls of the process chamber.
The present invention also provides a method of dissolving fuel in chopped nuclear fuel pins in an apparatus comprising a perforated sloping ramp contained within a process chamber for containing solvent for the fuel and a pulsation member which in use creates pulses in solvent in the process chamber, the perforations being designed to direct pulses of solvent along and up the ramp, the method comprising loading solvent into the process chamber, loading fuel pin pieces onto a lower region of the ramp and creating solvent pulses to transport the fuel pin pieces up the ramp to a discharge point where the cladding hulls are discharged from the ramp. The method may be performed in the reprocessing of nuclear fuel, the method further including reprocessing the dissolved fuel to form a fissile material optionally in the form of a fuel pellet, a fuel pin or a fuel assembly.